The present invention relates to piezoresistive pressure sensors and more particularly temperature compensated pressure transducers.
It is well known in a piezoresistive Wheatstone bridge having four equal piezoresistors of resistance RB of which two increase with positive strain and two decrease with an equal negative strain, that the change of voltage xcex94V across the bridge is given by:             Δ      ⁢              xe2x80x83            ⁢      V              V      B        =            Δ      ⁢              xe2x80x83            ⁢              R        B                    R      B      
and that             Δ      ⁢              xe2x80x83            ⁢      R        R    =      ε    ⁢          xe2x80x83        ⁢    GF  
where xcex5 is the strain, VB is the voltage applied across the bridge and GF is the gauge factor. It is also well known that the gauge factor decreases as a function of bridge temperature. The rate of change of gauge factor with temperature is usually referred to as TCGF or temperature coefficient of gauge factor. Thus, for a constant voltage applied across the bridge, the output will decrease as a function of temperature. It is also well known that resistance of the bridge elements increase as a function of temperature. The change of resistance with temperature is referred to as TCR or temperature coefficient of resistance. For highly doped P-type silicon, the TCGF is approximately xe2x88x922%/100xc2x0 F. to xe2x88x923%/100xc2x0 F. while the TCR is approximately +10%/100xc2x0 F. Referring now to FIG. 1, one way to make the output voltage more independent of temperature using a constant voltage source 15 is to place a temperature independent resistor RS in series with the bridge 2. Thus, as temperature increases the bridge resistance increases and more of the supply voltage appears across the bridge. For this case the bridge voltage VB is given by:       V    B    =                    R        B                              R          B                +                  R          S                      ⁢          V      0      
Thus, by appropriate choice of the ratio of RS to RB, the desired increase of bridge voltage with temperature can be obtained. This compensation scheme can be used in many applications. An example of such an apparatus and method is illustrated in U.S. Pat. No. 3,245,252, entitled xe2x80x9cTEMPERATURE COMPENSATED SEMICONDUCTOR STRAIN GAGE UNITxe2x80x9d issued Apr. 12, 1966, the entire disclosure of which is hereby incorporated by reference as if being set forth herein in it entirety. However, there are some instances when such an approach is unsuitable for certain needs. An example of such an application is where certain extremely tight specifications are needed, then the passive resistor alone can not accomplish the desired effect. This is especially true when the pressure transducer must be extremely precise at either the extreme cold end or extreme hot end of the operating temperature range of the device.
It is an object of the present invention to provide an improved temperature compensated transducer suitable for use in these types of applications.
A dielectrically isolated temperature compensated pressure transducer including: a wafer including a deflectable diaphragm formed therein, the diaphragm being capable of deflecting in response to an applied pressure, and the diaphragm defining an active region surrounded by an inactive region of the wafer; a plurality of dielectrically isolated piezoresistive elements formed on the active region of the wafer and coupled together to form a Wheatstone bridge configuration so as to cooperatively provide an output signal in response to and indicative of an amount of deflection of the diaphragm, the plurality of piezoresistive elements being undesirably operative to introduce an undesirable error into the output according to exposure of the wafer to an environmental condition; and, a dielectrically isolated resistor formed on the inactive region of the wafer and electrically coupled in series to the plurality of piezoresistive elements so as to at least partially compensate for the undesirable error.